For centuries, it was OK to use double and even triple negatives to show how really, really negative something was. Chaucer and Shakespeare did it all the time. (In Twelfth Night, for instance, Shakespeare has Viola using the triple negative “nor never none.”)

It wasn’t until the 18th century that the double negative was declared a no-no on the ground that one negative canceled out the other.

If you want your writing to be taken seriously, stay away from examples like “I can’t see nobody” or “He didn’t do nothing.” But a sentence like the one you mentioned (“He’s not unkind”) is perfectly good English.

A double negative comes in handy when you want to avoid saying something flatly or hurting somebody’s feelings. Instead of blurting out “Your blind date was a dog,” for example, you might use a double negative to say, “Your blind date was not unattractive.”